This invention relates generally to power control circuits, and more particularly to circuits for controlling the amount and fundamental frequency component of electrical power delivered to loads.
The increasing power capability of thyristor devices has led to significant changes in the techniques of power conversion and control. In particular, the thyristor has been widely used in apparatus which serves as a power controller between conventional utility power sources and loads which require variable voltage and frequency inputs. One such prior application of a power controller involves the use of back-to-back thyristors between a single phase AC power source and a load. By properly phasing the thyristor gate drive with respect to the input voltage waveform, control can be established over the amount of source voltage that is permitted to reach the load. Such controllers can be used with motor starters, light dimmers, variable speed apparatus control, electric heating, and similar applications. The basic single phase power controller can be modified to control three phase power sources and loads. In three phase applications, six thyristors are used, a back-to-back pair in each power line.
Both the prior single phase and three phase thyristor power controllers are controlled to produce load power at the same frequency as the input voltage. This feature limits the usefulness of the thyristor control technique when the load is a three-phase induction motor, for example. Increased torque and reduced rotor heating of the motor could be achieved at low speeds if the fundamental frequency component of the voltage applied to such a motor were reduced. The approach usually used for direct frequency reduction to gain such advantages is to provide the use of a conventional thyristor cycloconverter, but this approach generally requires 12 thyristors in addition to the six required for three phase control, increasing cost and complexity of the circuits.